Signed int in c language assignd bitfields but confusion

Question

Currently I am working on c and facing a confusion regarding signed int in structure and here I have given the example:

#include <stdio.h>
#include <string.h>

struct {
  signed int age : 4;
} Age;

int main( ) {

   Age.age = -8;
   printf("Age.age : %d\n", Age.age );

   return 0;
}

Here, I already have described the size of bits which int will occupy while storing the value. here I am assigning value -8 to age. so it will store value like 1000 for 8 and for -8 it should store like 11000 where left 1st bit is known as sign bit. So if int age have to store -8 it must have 5 bits but while I compile the given example it is not giving error and displays an output.

Please help me with my issues.

Check [this](http://stackoverflow.com/questions/1049722/what-is-2s-complement) answer .. signed values use 2's complement. 4 bits gives a range of `-8`-`7` .. For example, what output do you get when you do `Age.age = 8` ..? — txtechhelp, Jan 31 '17 at 05:59
So here's about signed bitfields: http://stackoverflow.com/a/4590032/2079103 — clearlight, Jan 31 '17 at 05:59
Your architecture is most likely using two's complement for number representation. For 4 bits then, the max signed integer is 0111 = 7 decimal, and the smallest is 1000 = -8 decimal. So with two's complement representation -8 fits in the four bits. — JimD., Jan 31 '17 at 06:03
See also: https://en.cppreference.com/w/c/language/bit_field — G. Sliepen, Aug 17 '19 at 13:37

score 2 · Accepted Answer · edited May 23 '17 at 10:29

2

Probably, integers are stored using 2's complement representation on your system.

In 2's complement, a 4-bit bitfield can hold a range of -8 through to +7. The bit representation 1000 will mean -8, and it is not possible to store +8.

Assigning an out-of-range value (such as +8 in this case) causes implementation-defined behaviour.

edited May 23 '17 at 10:29

Community

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answered Jan 31 '17 at 06:00

M.M

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I just gave you 2 compliments. – clearlight Jan 31 '17 at 06:05
Thank you for the great explanation in very short term. – Dhaval Mistry Jan 31 '17 at 06:26
Good understanding – ntshetty Jan 31 '17 at 07:53

Signed int in c language assignd bitfields but confusion

1 Answers1